part of the mesh become yellow when in sculpt mode
This is the new &quot;Face Sets&quot; feature, I think. It seems to be meant as an extension to the use cases currently covered by hiding and showing faces.
It does not seem to be particularly well documented, or to have much in the way of exposure in the user interface, at this point.
You probably did nothing wrong. I assume that the Boolean modifier probably automatically created two &quot;Face Sets&quot; corresponding to each mesh when they were joined.
To fix this problem, you can open the new &quot;Face Sets&quot; menu in the 3D view's Sculpt Mode header. The very last option says &quot;Randomize Colors&quot;, and will randomly reassign each region of your mesh a new colour. You can repeat this operation until the colours of your mesh are no longer yellow. (I hope you like purple, red, blue, or green.)
Alternatively, you could also try one of the options under &quot;Init Face Sets&quot;, or using the &quot;Draw Face Sets&quot; sculpting brush and holding CTRL from one area to paint over the rest of your mesh.